The ability to isolate DNA and clone this isolated DNA into bacterial plasmids has greatly expanded the approaches available to make viral vaccines. The method used to make the present invention involve modifying cloned DNA sequences by insertions, deletions and/or single or multiple base changes. The modified DNA is then inserted into a viral genome, and the resulting virus may then be used in a vaccine to elicit an immune response in a host animal and provide protection to the animal against disease.
Fowlpox virus (FPV) is a member of the poxviridiae family of viruses. There are two subfamilies in this classification, and they are differentiated based upon the host range (vertebrate or invertebrate) of the virus. Among the vertebrate poxviruses, there is serological cross reactivity to group specific antigens that has aided in classification of the viruses into six genera, and FPV has been placed in the avipoxvirus genera along with seven additional poxviruses that primarily infect birds. In general, poxviruses are the largest of the animal viruses and can be visualized with the light microscope. Under the electron microscope, the virus takes on a biscuit like or oval shaped appearance. The principal chemical components of the poxviruses are protein (90% by weight), deoxyribonucleic acid (DNA) (3%) and lipid (5%), but in FPV the lipid component is approximately 1/3 of the dry weight. Polyacrylamide gel electrophoresis (PAGE) of solubilized virions indicates that there are &gt;100 different proteins associated with the viruses that include: structural polypeptides, enzymes associated with translation of messenger ribonucleic acid (mRNA), enzymes involved in RNA synthesis, and enzymes associated with DNA replication. The genome of poxviruses consists of double-stranded DNA that varies in base composition (32% G+C to 64% G+C) and length (140 kilobasepairs [kb] to 280 kb for FPV) depending upon the individual virus. The complete nucleotide sequence of the vaccina virus genome has recently been determined, and most of the essential genes have been found to lie within the highly conserved middle region of the genome while nonessential functions seem to map nearer to the termini of the DNA. The poxviruses are unique in their propensity to replicate within the cytoplasmic space of the infected cell, and in the case of VV, mature virus particles are moved out of the assembly areas and into the periphery of the cell where additional membrane encapsulation occurs. With FPV, the assembled viral particles become associated with a dense viral-derived protein matrix that occludes the virus in the form of cellular inclusions that may help protect the virion from lytic activities. Depending upon the specific poxvirus and strain (from 1% to 30% of different mature VV strains) varying levels of mature virus can be found extracellularly, but the majority of the virus population remains associated with the cell at the end of the growth cycle.
Fowlpox is unique throughout the world, but because its host-range is limited to birds it is not considered to be a public health hazard. All chickens can be infected by the virus with a resulting decline in the growth rate of the bird and temporary decreases in egg production. Usually, transmission of FPV occurs through physical contact of injured skin, but there are reports that the virus is also transmitted via arthropod vectors. After an incubation period of four to ten days, the disease is typically manifested in the following ways: skin lesions in non-feathered areas, lesions of the nasal passages, and lesions of the mouth. A normal FPV infection usually lasts three to four weeks, and afterward the bird is conferred life-long immunity to the disease.
Currently, conventionally derived FPV vaccines are being used in commercial settings to provide protection to chickens and turkeys. Typically, the vaccine viruses are attenuated by serial passage in cell culture selecting for strains that have altered growth and/or virulence properties. The modified live vaccine is prepared by growth in vitro in chicken embryo fibroblast cells or by growth on the chorioallantoic membrane of the chicken embryo. The vaccine virus is typically given to birds subcutaneously.
The present invention concerns the use of FPV as a vector for the delivery of specific vaccine antigens to poultry. The idea of using live viruses as delivery systems for antigens (vectoring) has a long history that is associated with introduction of the first live viral vaccines. The antigens that were delivered were not foreign but were naturally expressed by the live virus in the vaccine. The use of viruses to deliver foreign antigens in the modern sense became possible with the advent of recombinant DNA methods. The vaccinia virus was the first such vector and various antigens from other disease-causing pathogens were used as the foreign antigens in a vaccine was created by genetic engineering. While the concept begins with these disclosures, the answers to more practical questions concerning what makes the best candidate viral vector and what constitutes the best foreign gene or gene to be delivered were not obvious. In answering these questions, details of the pathogenicity, site of replication or growth, the kind of elicited immune response, expression levels for the virus and foreign gene of interest, its suitability for genetic engineering, its probability of being licensed by regulatory agencies, etc. needed to be addressed.
The presently preferred method for creating a recombinant poxvirus uses a plasmid of bacterial origin that contains at least one cassette consisting of a poxvirus promoter followed by the gene of interest. The cassette(s) is flanked by poxvirus genomic DNA sequences that direct insertion of the gene of interest into the corresponding homologous nonessential region of the viral genome by homologous recombination. Cells are initially infected with the wild-type virus, and shortly thereafter the plasmid DNA is introduced into the infected cells. Since poxviruses have their own RNA polymerase and transcriptional apparatus, it is necessary that the gene of interest be regulated by a promoter of poxvirus origin. There are three characteristic poxvirus promoters that are differentiated based upon their temporal regulation of gene expression relative to the infective cycle of the virus: early, intermediate and late expression. Each promoter type can be identified by a typical consensus sequence that is approximately 30 bp in length and specific to each promoter type. In vaccinia virus, some viral genes are regulated by tandem early/late promoters that can be used by the virus to continually express the downstream gene throughout the infective cycle.
It is generally agreed that poxviruses contain non-essential regions of DNA in various parts of the genome, and that modifications of these regions can either attenuate the virus, leading to a non-pathogenic strain from which a vaccine may be derived, or give rise to genomic instabilities that yield mixed populations of virus. The degree of attenuation of the virus is important to the utility of the virus as a vaccine. Insertions or deletions which cause too much attenuation or genetic deletions which cause too much attenuation or genetic instability of the virus will result in a vaccine that fails to elicit an adequate immune response. Although several examples of deletions/insertions are known for poxviruses, the appropriate configuration is not readily apparent.
Thus far, gene expression from foreign genes of interest inserted into the genome of poxviruses has been obtained for five different pox viruses: vaccinia, canary pox, pigeon pox, raccoon pox and fowlpox. Vaccinia virus is the classically studied poxvirus, and it has been used extensively to vector foreign genes of interest (see U.S. Pat. Nos. 4,603,112 (1986) and 4,722,848 (1988). Raccoon pox (Esposito, et al., 1988) and Canary pox (Taylor, et al., 1991) have bene used to express antigens from the rabies virus. More recently, FPV has been used to vector a number of different foreign genes of interest, and is the subject of several published patent applications (EPA 0 284 416 (1988), PCT WO 89/03429 (1989), PCT WO 89/12684 (1989), PCT WO 91/02072 (1991), PCT WO 89/03879 (1989)). However, these publications do not teach the vectored antigen configuration, the FPV insertion sites, or the promoter sequences and arrangement of the present invention.
A foreign gene of interest targeted for insertion into the genome of FPV may be obtained from any pathogenic organism of interest. Typically, the gene of interest will be derived from pathogens that cause diseases in poultry that have an economic impact on the poultry industry. The genes may be derived from organisms for which there are existing vaccines, and because of the novel advantages of the vectoring technology the resulting FPV-derived vaccines will be superior. Also, the genes of interest may be derived from pathogens for which there are currently no vaccines but where there is a requirement for control of a disease. Typically, the gene of interest encodes an immunogenic polypeptide of the pathogen, and may represent surface protein, secreted protein or structural protein.
One relevant avian pathogen that is a target for FPV vectoring in the present invention is Infectious Laryngotracheitis virus (ILT). ILT is a member of the herpesviridiae family, and this pathogen causes an acute disease of chickens which is characterized by respiratory depression, gasping and expectoration of bloody exudate. Viral replication is limited to cells of the respiratory tract, where in the trachea the infection gives rise to tissue erosion and hemorrhage. In chickens, no drug has been effective in reducing the degree of lesion formation or in decreasing clinical signs. Vaccination of birds with various modified forms of the ILT virus derived by cell passage and/or tedious regimes of administration have conferred acceptable protection in susceptible chickens. However because of the degree of attenuation of current ILT vaccines, care must be taken to assure that the correct level of virus is maintained; enough to provide protection, but not enough to cause disease in the flock.
An additional target for the FPV vectoring approach is Newcastle disease, an infectious, highly contagious and debilitating disease that is caused by the Newcastle disease virus (NDV), a single-stranded RNA virus of the paramyxovirus family. The various pathotypes of NDV (velongic, mesogenic, lentogenic) differ with regard to the severity of the disease, as well as the specificity and symptoms, but most types seem to infect the respiratory system and the nervous system. NDV primarily infects chickens, turkeys and other avian species. Historically, vaccination has been used to prevent disease, but because of maternal antibody interference, life-span of the bird and route of administration, the producer needs to adapt immunization protocols to fit specific needs.
Marek's disease of poultry is a lymphoproliferative tumor producing disease of poultry that primarily affects the peripheral nervous system and other visceral tissues and organs. Marek's disease exists in poultry producing countries throughout the world, and is an additional target described by the present invention for a FPV-based vectored vaccine. The causative agent of Marek's disease is a cell associated gammaherpesvirus that has been designated as Marek's disease virus (MDV). Three classes of viruses have been developed as conventional vaccines for protecting chickens against Marek's disease: attenuated serotype 1 MDV, herpesvirus of turkeys (HVT), and naturally avirulent serotype 2 isolates of MDV. Protection obtained with these vaccines is principally directed toward the tumorigenic aspect of the disease. The occurrence of excessive Marek's disease losses in such conventionally vaccinated flocks has led to the requirement for forming admixtures of the various vaccine types. Such polyvalent vaccines while generally ore effective in disease control, complicate the vaccine regime.